Abstract Systemic lupus erythematosus (SLE) is a heterogenous systemic autoimmune disorder of unknown etiology. It has been more than a decade since it was first shown that a significant proportion of patients diagnosed with SLE express an ?interferon signature? in the peripheral blood mononuclear cells (PBMCs); that is upregulated levels of interferon-alpha (IFN?)-induced gene transcripts. Many cells can produce IFN?, but plasmacytoid dendritic cells (pDCs), in particular SiglecH+ pDCs, are particularly potent in part due to their high expression of endosomal toll like receptors (TLR7, TLR8 and TLR9), and the presence of pre- made IRF7; a key mediator of TLR7/9 induced signaling. Plasmacytoid DCs are found in elevated numbers in SLE patients and some mouse models of lupus, and recent data from us and others showed that depletion of SiglecH+ pDCs in mouse models of lupus ameliorates disease development. Despite these exciting results suggesting SiglecH+ pDCs as a new target for therapeutics, it remains to be determined how IFN? production is induced in SiglecH+ pDCs, what cells respond to SiglecH+ pDC-derived IFN? and consequently what pathological effects are driven by such cells. Based on the overwhelming evidence showing a correlation between pDCs, IFN? and SLE, we will test the hypothesis that SiglecH+ pDCs produce IFN? in response to endogenous activation of TLR7 signaling, and that IFN? act upon T cells, B cells and myeloid cells, all of which are responsible for specific measures driving lupus pathogenesis. In order to test our hypothesis, we propose a series of unique new approaches in which we take advantage of, and combine, a number of genetically modified, already existing, mouse models. First of all, we will identify critical target cells of pathogenic IFN? involved in, and necessary for, driving individual or overlapping symptoms of lupus-like disease in vivo. For these studies we have created cell-type specific IFN?-receptor deficient B6.Nba2 lupus-prone mice and will specifically identify the pathogenic effect of IFN? stimulated CD4+ T cells, B cells and LysM+ myeloid cells. Secondly, we will create mixed bone marrow chimera mice to test if TLR7 is directly acting in SiglecH+ pDCs instigating IFN? synthesis and driving lupus pathogenesis. For these studies we will take advantage of our B6.Nba2.BDCA2-DTR transgenic mice, in which SiglecH+ pDCs can be specifically ablated upon treatment with diphtheria toxin, and systemic TLR7- deficient mice. The completion of our proposed studies will provide critical new information regarding the relationship between SiglecH+ pDCs, IFN?, TLR7 and target cells of IFN? and lupus pathogenesis. It is our expectation that such knowledge will result in new therapeutic targets with the potential to help the millions of individuals suffering not just from SLE, but also from other autoimmune disorders (dermatomyositis, vitiligo, and Aicardi-Goutires syndrome etc.) in which IFN? has been shown to be pathogenic.